Carbon-carbon (C/C) composite materials offer excellent combinations of specific thermal conductivity and specific stiffness. Also, C/C offers low density, high stiffness, low coefficient of thermal expansion, zero outgassing in the space environment, and a unique high temperature capability. C/C composites graphitized to temperatures greater than 2200° C. provide high through-the-thickness (kz) conduction of 25 to 100 W/m-K (compared to kz=1-5 W/m-K in graphite/cyanate-ester composites), through appropriate choice of the starting matrix and potential fillers.
High performance aerospace vehicles and structures must be made with materials having high specific strength and specific stiffness. Carbon fiber composite materials are used in many lightweight structural applications. The performance and range of applications of these materials are limited, however, by their relatively high susceptibility to impact damage. For example, the space shuttle Columbia accident is believed to have been caused by impact damage to the reinforced carbon-carbon (RCC) leading edge thermal protection system.
The brittle nature of the RCC may be due to the conventional 2D or 2.5D carbon fiber ply architecture. The RCC material used in the space shuttle is fabricated from a precursor 2D woven carbon fiber (rayon based) fabric stacked in 0° and 90° oriented layers. The laminate is 19 layers thick, with the outer two or three layers infused with silica to produce a silicon carbide layer upon subsequent high temperature processing. The SiC outer layer prevents oxidation to allow the RCC structure to survive the high re-entry temperatures encountered by the space shuttle. A micrograph of a cross-section of this RCC material shows significant residual porosity throughout and microcracks in the SiC surface layer. The carbon fiber reinforcement layers exhibit a waviness due to the high crimp angle at fiber crossing points that is inherent in regular thickness woven carbon fiber plies.
Spreading of carbon fiber tows to make thin plies has been achieved by automated tow spreading technology. See U.S. Pat. No. 6,032,342. Thin plies of consistent quality have recently become commercially available, for example, from Itochu in Japan. Plies are available down to a thickness of 1.5 mils, which is five times thinner than the typical 7.5 mil thick standard plies. The plies are available as a unidirectional fabric or a woven fabric formed of ribbon tapes, in various widths, and in long continuous lengths. Composite materials manufactured from thin ply carbon fiber preforms exhibit surprising improvements in mechanical properties over conventional ply composites of identical overall thickness. Tensile strength is increased 30%. Fatigue life is increased more than an order of magnitude. Stiffness and reduction in delamination damage are increased.
Using carbon nanotubes (CNTS) alone as reinforcement to enhance the mechanical properties of a material is challenging. In particular, dispersing CNTs to obtain uniform distributions in a composite is difficult. Furthermore, the high volume fraction loadings (on the order of 60 vol %) required significantly increases the viscosity of the fresh resin/CNT mixture. The use of CNTs is also costly, particularly at such large volumes.